Site-specific mapping of transition metal oxygen coordination in complex oxides. S. Turner*, R. Egoavil, M. Batuk, A. A. Abakumov, J. Hadermann, J. Verbeeck, and G. Van Tendeloo EMAT, University of Antwerp, B-2020 Antwerp, Belgium E-mail:
[email protected]
We demonstrate site-specific mapping of the oxygen coordination number for transition metals in complex oxides using atomically resolved electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Pb2Sr2Bi2Fe6O16 contains iron with a constant Fe3+ valency in both octahedral and tetragonal pyramidal coordination and is selected to demonstrate the principle of site-specific coordination mapping. Analysis of the site-specific Fe-L2,3 EELS data reveals distinct variations in the fine structure that are attributed to Fe in a six-fold (octahedron) or five-fold (distorted tetragonal pyramid) oxygen coordination. Using these variations, atomic resolution coordination maps are generated that are in excellent agreement with simulations.
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In complex transition metal oxides the bonding and electronic state of the transition metal cations, i.e. the oxygen coordination, spin state and oxidation state, is of fundamental importance. The spin state, valency and oxygen coordination of the transition metal cations are all intricately related to the structural, electronic, magnetic, catalytic and ionic transport properties of the oxide materials.1-3 Information on the oxygen coordination of the transition metal cations (coordination number, bond lengths, bond angles) is conventionally available from various diffraction or spectroscopic methods such as X-ray/neutron/electron diffraction, extended X-ray absorption fine structure (EXAFS), Mössbauer spectroscopy or their combinations4. However, the changes in the crystal- and electronic structure are often needed to be monitored in materials applications with high spatial resolution, which is not available for the above mentioned methods.5,6 For example, valency changes at surfaces or under metallic surface particles are of vital importance for many catalytic processes while coordination or spin changes at defects like twin boundaries, grain boundaries and interfaces can greatly affect the electronic, optical and transport properties of bulk materials and thin films.7-9 Atomic resolution elemental mapping by means of spatially resolved electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EELS and STEM-EDX) has become a well-established technique over the past years.10-15 STEM-EDX appears to allow more straightforward data acquisition and interpretation as compared to EELS. However, recent work has shown that the interpretation of atomic resolution data needs to be combined with simulations for an accurate interpretation in both cases.16 In addition, with the current commercial spectrometer technology, structural information like bonding or coordination cannot be obtained from X-ray spectra.17 Therefore combining the sensitivity of EELS to valency, coordination and spin state through the EELS fine structure (ELNES) with the atomic resolution capabilities of a STEM remains the most direct method to obtain atomic resolution structure information. Oxidation state mapping at atomic resolution was recently demonstrated, using the correlation between ELNES and valency.7,18 The main issues hindering atomic resolution valency and bonding measurements are poor EELS signal-to-noise ratios due to the need for simultaneous high spatial and energy resolution of the instrument, and problems of signal intermixing due to elastic scattering in the case of “thick” crystals.19 These problems are worsened further as, in general, changes in the fine structure of the L2,3 edge of transition metals due to bonding or coordination are supposed to be far more subtle than changes related to valency.20,21 This makes studies where bonding and coordination have been mapped at atomic resolution rare, and in most cases the change in bonding coincides with a change in valency which also affects the EELS fine structure.12,22-24 In recent work by our group, distinct changes in fine structure of the Fe L2,3 and Co L2,3 edges were found between Fe and Co in octahedral and in tetrahedral layers in Ca2FeCoO5 brownmillerite using atomic resolution STEMEELS. This experiment demonstrated the problems of atomic resolution oxygen coordination 2
mapping: even though the octahedral and tetrahedral signatures could be measured at each distinct plane, the signal to noise ratio and signal intermixing in the experiments did not allow for a column by column investigation.25 As stated above, column by column information is of crucial importance when studying e.g. point defects or surface sites. In this work, we demonstrate the sensitivity of transition metal L2,3 edges to the change in local oxygen coordination, which can be used to map out the coordination at atomic resolution. Pb2Sr2Bi2Fe6O16, a perovskite-based material with a structural incorporation of crystallographic shear (CS) planes was selected for the experiment. This type of complex ferrites, combining the magnetic transition metal cations at the B-position of the perovskite structure with the lone pair A-cations, potentially demonstrate a combination of ferroic properties such as antiferroelectric and antiferromagnetic orderings found in (Pb,Bi)1-xFe1+xO3-y perovskites.26 Within the material, the oxygen coordination of Fe changes from an octahedral (6-fold) coordination in perovskite blocks to a distorted tetragonal pyramidal (5-fold) setting at the CS planes, while keeping the Fe3+ valency constant. For the AnBnO3n-2 (A = Pb, Bi, Ba, Sr, B = Fe, Ti, Sn) family of the perovskite-based oxides with the perovskite blocks separated by the CS planes the presence of the iron atoms in two distinct 5- and 6-fold coordinations as well as the constant Fe3+ oxidation state were proven by neutron powder diffraction and Mössbauer spectroscopy.27-29 Pb2Sr2Bi2Fe6O16 has an orthorhombic crystal structure with lattice parameters a = 5.7199(1) Å, b = 3.97066(7) Å, c = 32.5245(8) Å and belongs to the n = 6 member of this family. The fact that Pb2Sr2Bi2Fe6O16 is a mixed coordination/single-valency compound makes it an ideal candidate to demonstrate pure coordination mapping at atomic resolution; EELS fine structure variations between crystallographically distinct Fe positions can in this way unequivocally be assigned to coordination changes, and not to e.g. valency variation. The sample with composition Pb2Sr2Bi2Fe6O16 was synthesized using a high temperature solid state reaction of PbO, SrCO3, Bi2O3 and Fe2O3. The starting materials were mixed in the molar ratio 1:1:0.5:1.5, thoroughly ground, pressed into a pellet and annealed in air at 750°C for 24 h, 850°C for 24 h and 900°C for 15 h with intermediate regrinding. The sample was prepared for TEM investigation by crushing the powder, dispersing it in ethanol and depositing the dispersion onto a holey carbon grid. The sample was investigated using a FEI Titan “cubed” microscope equipped with a probe corrector and a monochromator, operated at 120 kV. The microscope was operated in STEM mode using a convergence semi-angle α of 18.5 mrad. The monochromator was excited to provide an energy resolution of ~ 250 meV, and the energy slit was chosen to provide a beam current close to 60 pA for spectroscopy, while keeping acceptable spatial resolution (probe size of approximately 1.5 Å). The HAADF inner collection semi-angle and spectrometer acceptance semi-angle β was 160 mrad. The acquisition time per pixel was 80 ms and was chosen to avoid beam damage and provide the best possible signal-to-noise ratio. The iron elemental map in Figure 2 was generated by plotting the intensity under the background subtracted L3 edge using a 9 eV energy window. The maps in Figure 3 3
were generated by back-fitting the 6-fold and 5-fold coordinated Fe EELS reference components from Figure 2 in a linear combination to the acquired EELS data cube using the EELSmodel software package.30,31 A power-law background (A.E-r) model for the EELS data was used in the fit. When filtered, a 3x3 light low-pass filter was used in the Digital Micrograph software package. The image simulations were performed with the STEMSIM software package, a MATLAB based image simulation program capable of handling the complex interplay between elastic and inelastic scattering in the double channeling approximation.32 Elastic scattering was simulated with a Bloch wave approach (max wave vector 2.0 Å-1) at a total thickness of 10 nm using a unit cell sampling: 135*24 pixels per unit cell. Source size broadening was taken into account using a Gaussian with 0.7 Å FWHM and a Lorentzian with 0.2 Å FWHM.33 Inelastic scattering was simulated using a relativistically corrected Dipole approximation with a hard Bethe ridge cutoff. Figure 1a shows a high-angle annular dark field (HAADF) STEM image of the Pb2Sr2Bi2Fe6O16 sample along the most informative [010] zone axis orientation. In the image the a and c directions are indicated by arrows and the unit cell is marked by a white rectangle. The structural repetition of the perovskite blocks containing Bi, Sr as A cations and 6-fold coordinated Fe as B cation and the crystallographic shear planes with Bi, Pb atomic pairs alternating with pairs of 5-fold coordinated Fe is immediately apparent. The image is taken with the electron monochromator excited, providing an energy resolution of approximately 250 meV. Under these conditions the spatial resolution is still sufficient to image all the structural details of the compound. The crystal surface is extremely clean, which is crucial for high resolution EELS experiments; and whenever present the amorphous surface layer is below 1 nm. Sample thickness is also likely to be a crucial factor in this type of experiment. As the EELS signal intermixing due to elastic scattering in the crystal increases with sample thickness, a very thin sample region close to the crystal surface was selected for investigation. A Bloch wave HAADF image simulation for a 10 nm thick crystal is inserted into Figure 1a, and agrees remarkably well with the experimental image. The projected structural models for the [100] and [010] zone axis orientations are displayed in (c) and further elucidate the coordination of Fe within the structure. The atomic resolution EELS data acquired at high energy resolution are plotted in Figure 2. To acquire the spectroscopic data needed for coordination mapping, the spectrum imaging (SI) technique was adopted.17 In this technique, the electron probe (in our setup the probe has an approximate size of 1.5 Å) is scanned over the sample and an EELS spectrum is acquired in each point together with a highangle annular dark-field signal as image reference. The overview image in Figure 2a shows the region selected for spectrum image acquisition. The simultaneously acquired HAADF image is displayed in Figure 2b. The Fe L2,3 edge was acquired to investigate local changes in Fe coordination. The L3 and L2 “white lines” arise from transitions of 2p3/2 → 3d3/23d5/2 (L3) and 2p1/2 → 3d3/2 (L2) and are known to be sensitive to valency and coordination as their intensity is related to the density of unoccupied states in the 3d bands.20,34 Using a simple integration window placed over the background-subtracted 4
L3 edge in the EELS datacube, an Fe elemental map was generated, and is plotted in Figure 2c. At this point it is important to note that no post-processing such as principle component analysis (PCA/weighted PCA35) of the data was performed. All analyses were carried out on raw data. Previous studies have shown that PCA treatment of atomic resolution data can mask small details in the acquired EELS data.36 The distinction between the 5-fold coordinated Fe3+ atomic pairs in the CS plane structures and the 6-fold coordinated Fe3+ B cations in the perovskite blocks is immediately apparent. Simple inspection of the data summed over Fe positions in the perovskite blocks (blue region) and the Fe CS plane structure positions (red region) reveals distinct changes in the fine structure for the two types of Fe coordination. The data collected from the octahedral Fe3+ sites (blue) displays a clear splitting of the L3 peak, which results from the split of the energy levels in the Fe 3d unoccupied states into t2g and eg levels. The pre-peak to L3 at 708 eV is associated with transitions from 2p3/2 → t2g. The main L3 maximum, associated with a transition from 2p3/2 → eg is present at 709.5 eV. The fine structure shape and edge onset values are therefore both in full agreement with literature data for Fe3+ in an octahedral coordination.20 The data collected from the tetragonal pyramidal Fe3+ sites shows other distinct features. A first observation is that, even though the ELNES shape of the peak varies with respect to the octahedral data, the edge onset remains the same, confirming that the valency of Fe is not changing from one crystallographic site to another.19 The L3 maximum is shifted by 0.3 eV to lower energy. At the same time, the pre-peak to the L3 is subdued due to the loss in symmetry and/or a decrease in the crystal field splitting at these crystallographic positions, similar to the case of 4-fold coordinated Fe3+.4,25 The sum of both signals yields the bulk Fe L2,3 edge (black spectrum). In all, it is clear that even though the coordination of the Fe cations changes by only a single oxygen atom, clear differences are present in the Fe L2,3 ELNES signatures. In Figure 3, the internal reference data for Fe in the two distinct coordinations are fitted to the entire acquired EELS datacube, following the procedure from earlier work on atomic resolution valency mapping.7,18,30,31 Fitting the internal octahedral and tetragonal pyramidal reference data to each acquired spectrum results in Fe oxygen coordination maps with the weight of each component to be generated. The results of the fit are displayed in Figure 3a. It can be seen that each component peaks at the correct atomic columns. The 6-fold coordinated Fe positions in the perovskite blocks peak in the map of the octahedral coordination, while the 5-fold coordinated Fe positions peak in the map of tetragonal pyramidal coordination, even resolving the Fe3+ atomic pairs that are only separated by 2.3 Å in this projection . In order to confirm the direct interpretation of the generated coordination maps, detailed image simulations were carried out. These simulations are plotted in Figure 3c and agree very well with the experimental maps. When a light smoothing is applied to the experimental data (Figure 3b), the similarity between experiment and simulation is even more striking. To judge the match between experiment and simulation more quantitatively, a line profile through both is overlaid in Figure 3d for both the octahedral and the tetragonal pyramidal maps. Both simulations and experiment 5
indicate that even though the crystal selected for experiments was thin and almost free of amorphous surface, beam spreading due to elastic scattering and inelastic delocalisation is still a significant effect which leads to a large part of the EELS signal leaking into the background. In all, these maps clearly demonstrate that oxygen coordination of the transition metal cations can be mapped in complex oxide structures down to atomic resolution. Even though the coordination of the two distinct Fe positions only differs by a single oxygen atom, and the resulting ELNES variations in the Fe L2,3 edge are small, the high signal-to-noise ratio in the individual spectra allow the coordination to be mapped atomic column by atomic column. In conclusion, we have demonstrated the principle of oxygen coordination mapping by atomically resolved, high resolution EELS in an aberration-corrected electron microscope, through the example of single valency/mixed coordination compound Pb2Sr2Bi2Fe6O16. Detailed analysis of the Fe-L2,3 edge showed subtle and distinct variations in the fine structure that could be attributed to Fe3+ in tetragonal pyramidal or octahedral coordination; the results are in excellent agreement with literature. Using the spectral data as an internal reference, the coordination of the Fe cations in the compound could be mapped column by column. These experiments open the gate for coordination determination at surfaces, defects and grains boundaries in a plethora of complex oxide materials that have been out of reach in the past due to the lack of suitable analysis techniques for the study of valence and coordination as input for structure solving.
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FIGURES
Figure 1: Pb2Sr2Bi2Fe6O16 structure. (a) HAADF-STEM image along the [010] zone axis. The unit cell is indicated by the white rectangle. The inset image simulation for a 10 nm thick crystal matches well with the experiment. (b) Fourier transform of the image in (a), demonstrating information transfer beyond 6.0 nm-1 (reflections marked by red circles). (c) Structural model of the structure along the [100] and [010] zone axis orientations. The 6-fold (octahedral) coordinated Fe species are rendered in blue, the 5-fold coordinated (tetragonal pyramidal) Fe species in red, Pb/Bi columns in green, Bi/Sr columns in violet.
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Figure 2: Site-specific ELNES investigation. (a) HAADF-STEM overview image of the 44*25 pixel spectrum image region, indicated by the white rectangle. (b) Simultaneously acquired HAADF-signal (c) Fe map, generated by integrating a 9 eV wide energy range under the background subtracted Fe L3 edge in the spectrum image. (d) Summed Fe L2,3 edges from the 6-fold coordinated Fe sites (blue spectrum, blue region indicated in in (b) and (c)), 5-fold coordinated Fe sites (red spectrum, red region
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indicated in (b) and (c)), from the sum of both regions (black spectrum) and a single pixel spectrum from an 6-fold octahedral position (grey spectrum).
Figure 3: Column by column coordination mapping. (a) EELS maps obtained by point by point fitting of the internal reference Fe L2,3 data for 5-fold and 6-fold coordinated iron to the EELS datacube. The colour overlay displays the octahedral iron columns in blue and the tetragonal pyramidal iron columns in red. (b) EELS maps after low-pass filtering. (c) Simulated inelastic maps. (d) Line profiles over the low-pass filtered data from the positions indicated by the arrows in (b) at 4 pixel width, with overlaid line profiles from the simulated maps. ACKNOWLEDGEMENTS D. Batuk is acknowledged for fruitful discussions. ST gratefully acknowledges the Fund for Scientific Research Flanders (FWO). Part of this work was supported by funding from the European Research Council under the FP7, ERC grant N 246791 COUNTATOMS and ERC Starting Grant N 278510 VORTEX. The EMAT microscope is partially funded by the Hercules fund of the Flemish Government. The authors acknowledge financial support from the European Union under the Framework 7 program under a contract for an Integrated Infrastructure Initiative (Reference No. 312483 ESTEEM2). This work was funded by the European Union Council under the 7th Framework Program (FP7) grant nr NMP3-LA-2010-246102 IFOX. REFERENCES 1. S. Stolen, E. Bakken, and C. E. Mohn, Physical Chemistry Chemical Physics 8(4), 429 (2006). 2. N. Erdman, O. Warschkow, M. Asta, K. R. Poeppelmeier, D. E. Ellis, and L. D. Marks, Journal of the American Chemical Society 125(33), 10050 (2003). 9
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